1. Field of the Invention
The present invention relates to an electronic watermark embedding apparatus and an electronic watermark embedding method for embedding information into a document image using an electronic watermark technique.
2. Description of the Related Art
A technique for embedding information into a document image using an electronic watermark technique is disclosed in Japanese Patent No. 3628312. This document discloses a method of arranging pixels (dots) on a background of a document according to a predetermined method so as to be capable of adding confidential information or the like to the document. In this method, since a watermark is deleted from a portion where a character or the like of the document is overlapped with the watermark, the information on this portion cannot be read. However, redundancy is given to the watermark information so that the correct information can be entirely read. For example, the redundancy is given by repeatedly embedding a dot pattern showing identical information.
However, when a wide range of a document background is screened or the document includes large photograph or illustration, the information is broken up to the redundancy of the watermark information, and thus a watermark which can be sufficiently read cannot be embedded. A technique which copes with such a problem includes a technique disclosed in Japanese Patent Application No. 2005-326371 filed by the applicants of the present invention. This is a method of detecting the screening on a document image and replacing the detected screening by a watermark dot pattern having the same pixel density so as to be capable of embedding a readable watermark into an image with a large screened area. Concretely, dots of the screening are detected and periodicity of the dots is measured so that the screening and its pixel density are measured.
Since the method disclosed in Japanese Patent Application No. 2005-326371 is used for the screening, the method can be applied to a screened portion on a document image and an image binarized by a dithering method or the like. This method, however, cannot be applied to uniform half tone and a gradation portion on a multiple-valued image.